Networked motorized drive roller conveyor

ABSTRACT

A motorized drive roller conveyor includes an upstream zone and a downstream zone, with each zone having a drive roller, an idler roller that is driven by the drive roller, and a sensor. The upstream zone and the downstream zone are controlled by a card, which measures a gap between a first item on the conveyor and a second item on the conveyor by beginning a counter when a trailing edge of the first item passes the sensor of the upstream zone and stopping the counter when a leading edge of the second item passes the sensor of the upstream zone to generate a counter value. If the first item is stopped in the downstream zone, the card of the upstream zone causes the drive roller of the upstream zone to advance the second item into the downstream zone for a distance derived from the counter value before stopping the transportation of the second item.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to a networked motorized driveroller conveyor.

Conventional conveyor control systems utilize a central ProgrammableLogic Controller (“PLC”) mounted in a central control panel. This typeof system requires control devices, e.g., photo-eyes, solenoid valves,and motor starters, to be terminated at the main control panel. Atypical control system utilizing a PLC requires several hundred toseveral thousand feet of control wiring, which requires significanttime, labor and money to route and terminate the wiring. In addition,the PLC requires specialized knowledge, e.g., knowledge of ladder logic,and familiarity with the many different interface requirements unique toeach manufacturer's product. Moreover, since there is a centralcontroller, response time with respect to the control devices may beincreased because the single processor must account for all operationsoccurring within the system.

A Motorized Drive Roller (“MDR”) is a conveyor roller with an integratedmotor. An MDR is typically configured to drive a plurality of idlerrollers, usually by way of urethane belts or chains. The MDR and idlerrollers thus define a zone. There is typically one MDR per zone.

For conveyor systems that utilize MDR technology, networked motorcontrollers are frequently used to handle basic transport, diversion,and accumulation tasks to move items through a conveyor system. Thesemotor controllers are mounted in close proximity to the MDR rollers anddirectly interface with the product sensors associated with each MDR. AnMDR conveyor system has many advantages over other conveyortechnologies, such as lower power consumption, noise reduction, and adecreased need for maintenance.

Unlike conventional control systems, modular distributed controls don'trequire hundreds to thousands of feet of wiring from a centralized PLCto each device in the system. With controls located near the controldevices, wiring and wiring labor may be reduced. This shortens the timeto complete implementation and provides a parallel control capabilitythat minimizes response time issues common to conventional PLC basedsystems. Because the system is not limited by the speed of a centralprocessor in a PLC, the system can grow without worry of overtaxing acentralized controller. Testing and start-up time is also reduced, asvarious segments of the system can be installed and tested independentof other segments.

It is common practice in existing conveyor systems that use MDR rollerconveyor along with associated motor controllers to utilize smaller,localized PLC's distributed throughout the system to handle conveyoroperations such as diverting, bar code scanning, RFID communication,label applications, etc. These peripherals typically communicateserially (RS232 or RS485) or via a network protocol such as Ethernet.Having multiple PLCs in this environment creates other undesirableissues as single point diagnostics are difficult to implement.

The networked, distributed control system of the present inventionprovides localized controls for various operations, e.g., diverting, barcode scanning, RFID transactions, labeling, etc. The inventive controlsystem can also handle the basic MDR conveyor drive and accumulationresponsibilities, which greatly reduces the wiring needed for thesystem, implementation time, and cost while maintaining a centralizeddiagnostic capability. Additional capability to allow localizedprogramming as well as status and diagnostics capability are additionalbenefits of the inventive control system. Eliminating the need for PLCsand associated ladder logic is a further benefit of the inventive designbecause it reduces the complexity of installation, operation andmodification of the control system and corresponding conveyor system.

As noted above, MDR conveyor systems have many advantages over otherconveyor technologies, such as lower power consumption, noise reduction,and less maintenance. However, prior to the present invention, MDRsystems lacked the ability to control the size of gaps between items onthe conveyor system. Prior art systems are also limited to transporting“Items” that are shorter in length than a single “Zone”.

Existing technology is typically marketed as “Zero pressureAccumulation” conveyor technology, as items on the conveyor are allowedto accumulate with one item per zone. As such, with control technologycurrently marketed, items accumulate with varying gaps between items,based on the length of the items.

In accordance with one aspect of the inventive conveyor control system,a gap control arrangement is used to control gaps between items on theconveyor system. The elimination of gaps between items on the conveyorsystem is desirable, in that gaps reduce the number of items that can beaccumulated on the conveyor, providing lower accumulation efficiency.Thus, reducing or even eliminating gaps between items helps to maximizethe accumulation efficiency of the conveyor system. The gap controlsystem incorporates the use of MDR technology and its desirableattributes while at the same time providing the operator with theability to control the size of the gaps between items on the conveyorsystem.

In one embodiment of the invention, a networked motorized drive rollerconveyor includes a plurality of motorized drive roller assemblies,where each assembly comprises a zone. The conveyor has a plurality ofnetworked cards, with each card controlling a pair of adjacent zones.The conveyor further has a plurality of sensors for detecting items onthe conveyor, with each sensor corresponding to a zone. For a pair ofadjacent zones, the corresponding networked card measures a gap betweenconsecutive items on the conveyor by beginning a counter when a trailingedge of a first item passes the sensor of an upstream zone within thepair of zones, and stopping the counter when a leading edge of thesecond item passes the sensor of the upstream zone within the pair ofzones, to generate a counter value. If the first item is stopped in thedownstream zone, the networked card causes the motorized drive rollerassembly of the upstream zone to move the item into the downstream zonea distance derived from the counter value before stopping the movementof the second item.

In another embodiment of the invention, a motorized drive rollerconveyor includes an upstream zone and a downstream zone. Each zone hasa drive roller, an idler roller that is driven by the drive roller, anda sensor. A card controls the upstream zone and the downstream zone. Thecard measures a gap between a first item on the conveyor and a seconditem on the conveyor by beginning a counter when a trailing edge of thefirst item passes the sensor of the upstream zone and stopping thecounter when a leading edge of the second item passes the sensor of theupstream zone to generate a counter value. If the first item is stoppedin the downstream zone, the card of the upstream zone causes the driveroller of the upstream zone to advance the second item into thedownstream zone for a distance derived from the counter value beforestopping the movement of the second item.

In yet another embodiment of the invention, a method for controlling agap between items on a motorized drive roller conveyor includes thesteps of: (a) detecting a trailing edge of a first item at apredetermined location in a first zone on a conveyor system; (b)beginning a counter once the trailing edge of the first item passes thepredetermined location; (c) stopping the counter upon the firstoccurrence of the following:(i) a leading edge of a second item isdetected at the predetermined location, or (ii) the first item isstopped in a downstream zone adjacent the first zone; and (d) generatinga counter value.

These and other aspects and advantages of the present invention will bemade apparent from the following description taken together with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention is illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout.

In the drawings:

FIG. 1 is an isometric view of a networked MDR conveyor in accordancewith the present invention;

FIG. 2 is schematic of a card used in connection with the networked MDRconveyor of the present invention;

FIG. 3 is a representation of a physical embodiment of a card used inconnection with the networked MDR conveyor of the present invention;

FIG. 4 is another isometric view of the networked MDR conveyor of FIG. 1;

FIG. 5 is a schematic side elevation view of a networked MDR conveyor ofthe present invention;

FIG. 6 is an enlarged partial isometric view of the networked MDRconveyor of FIG. 1 ;

FIG. 7 is a side view of the networked MDR conveyor of FIG. 1illustrating gaps between items on the conveyor;

FIG. 8 is a top view of the networked MDR conveyor of FIG. 1 ;

FIG. 9 is a side isometric view of a second embodiment of a networkedMDR conveyor in accordance with the present invention;

FIG. 10 is an enlarged partial side isometric view of the networked MDRconveyor of FIG. 9 ;

FIGS. 11A-11G are schematic views of various conveyor configurationsthat may be integrated into a networked MDR conveyor in accordance withthe present invention; and

FIG. 12 is a flow chart illustrating a method of determining the size ofa gap between items in the networked MDR conveyor in accordance with thepresent invention.

DETAILED DESCRIPTION

FIG. 1 shows a portion of the conveyor system 10 of the presentinvention that incorporates control cards 20 in accordance with thepresent invention. Generally speaking, a number of items 11 aresupported on the conveyor 10, which includes a plurality of rollers 13,including MDRs and corresponding idler rollers. The conveyor system 10further includes a plurality of sensors 14 that detect items 11 as theitems 11 move along the conveyor system 10. The conveyor system 10 isdivided into zones 15. Each zone 15 is defined between adjacent sensors14. Additionally, each zone 15 corresponds with an MDR/idler rollerassembly. In the inventive system, each zone 15 is shorter than thelength of the respective items 11 being conveyed by the conveyor system10.

FIGS. 2 and 3 illustrate a networked, distributed MDR control card 20with local control capabilities. Accordingly, the inventive card 20eliminates the need for a central PLC and reduces and/or eliminates theneed for wiring associated with a system using a PLC. As a result,implementation and installation costs of a conveyor control system 10incorporating the MDR control cards 20 of the present invention may alsobe reduced while, at the same time, providing a more efficientinstallation and implementation process. Still further, the card 20 ofthe present information is programmable, and therefore is able to bereprogrammed depending on its desired utilization.

In the illustrated embodiment, the card 20 contains drivers 22, e.g., ona microprocessor, for one or more MDR rollers, which preferably featurea dual brushless motor. More specifically, for zero-pressureaccumulation mode of operation, e.g., one in which the items on theconveyor are not intended to contact one another, each card 20 willservice two zones 15. In other words, the card 20 will contain driversfor respective MDR rollers in adjacent zones 15. The drivers are adaptedto support any variation of MDR roller, e.g. commonly available MDRrollers, e.g., those ranging from 22 watts to 50 watts, may be used.

The card 20 of the present invention includes a plurality of Inputs andOutputs (“I/O”) 23 for interfacing with various components that aretypically utilized in conveyor assemblies, e.g., limit switches,solenoid valves, motor contactors, alarms/buzzers, and status beacons.

The card 20 further includes a plurality of photo sensor input jacks 24that connect with sensors 14, which, as discussed above, detect items 11that move along the conveyor system 10. For example, four such jacks 24are shown on the block diagram in FIG. 2 ; see also, FIG. 3 . The card20 also includes one or more network jacks 25 to provide networkcommunications capabilities for connection to a supervisory controlcomputer (e.g., a personal computer or industrial computing device). Theillustrated embodiment incorporates a CAN network using Can-Open orDevice-Net protocols, although Ethernet or serial communications couldalso be utilized. In this arrangement, any card 20 in the conveyorsystem 10 can communicate over the network to any other card 20 attachedto the network or to a supervisory control element, such as a PC orindustrial controller. Accordingly, system-wide changes, e.g., relatingto product volume, conveyor speed, etc., can be implemented using anycard 20 on the network.

The card 20 further includes a serial I/O port 26 to facilitatecommunications with external peripheral devices such as bar codescanners, RFID reader/writer devices, label applicators, in motion weighscales, or other serial devices.

The card 20 may also feature an operator interface display 27. Forexample, the display may include button switches for interfacing withthe module, a high intensity beacon for fault and error notification,and a display element that provides textual and/or graphical informationto the operator. An operator interface display allows parameters such asroller speed, acceleration and deceleration rates, delays, etc. to beset locally from any card 20, rather than from a central point such as aPLC.

In this description, the term “card” is used to describe item 20, whichcontrols operation of adjacent zones in conveyor system 10. It isunderstood that the term “card” is used for convenience, and that item20 may be any satisfactory control device that includes the features andfunctionality to connect to the sensors and drive rollers of theadjacent zones of conveyor system 10 and to control their operation. Theterm “card” is not intended to denote any particular structural orphysical characteristics of the control device.

The inventive conveyor system 10 improves upon existing designs byentirely eliminating the need for a PLC controller for normal conveyoroperations. The conveyor system 10 allows additional functions to beimplemented seamlessly without the need for PLCs or any knowledge ofladder logic on the part of the operator or system installer.

FIG. 11 shows typical configurations that are included in conveyorsystems. While typically controlled by a PLC, in the inventive conveyorsystem 10 these operations are controlled locally by any one or group ofcards 20 in the conveyor system 10. Such typical configurations includebut are not limited to the following examples. For example, FIG. 11Ashows a zone accumulator and FIG. 11B shows a back-to-back accumulator.

One configuration is an interface with an in-motion weigh scale as shownin FIG. 11C, which allows items 11 to be singulated and transportedsingly over an in-motion weigh scale. An identification device, such asa bar code scanner or RFID device, is connected directly to a card 20and provides the method of identification of the item 11. The weight ofthe item 11 is compared locally, i.e., at a card 20, with an expectedresult and a decision is made locally, i.e., at a card 20, to allow theitem 11 to continue along the conveyor system 10 or to be diverted offto a second conveyor in the event the weight of the item 11 is notwithin a defined tolerance. The I/O 23 on the card 20 will be utilizedto communicate with the in-motion weigh scale.

Another type of configuration is an interface with a label applicator or“print and apply” label applicator as shown in FIG. 11D. Thisconfiguration allows items 11 to be singulated and transported singlypast a labeler, such as an in-motion labeler, static label only, or“print and apply” labeler. An identification device, e.g., a bar codescanner or RFID device, is connected directly to the card 20 andprovides the method of identification of the item 11. The informationrequired on the label is determined locally on the card 20. The label isprinted (if necessary) and applied and then the item 11 is passed on tothe next conveyor zone 15. The I/O 23 on the card 20 will be utilized tocommunicate with the printer/applicator.

Another type of configuration is a 90 degree transfer as shown in FIG.11E. This pre-programmed operation will facilitate an item 11 transferfrom one conveyor zone 15 to another completing a 90 degree right angletransfer. A corresponding card 20 processes the timing required toensure that the item 11 is completely transferred prior to initiatingthe next transfer of an item. The card 20 further processes anyadditional input/output required to signal the transfer device.

Yet another configuration is a “+” transfer as shown in FIG. 11F. Thispre-programmed operation will facilitate an item transfer from oneconveyor zone to one of three possible divert points. An identificationdevice, e.g., a bar code scanner or RFID, is connected directly to acorresponding card 20 and provides the method of identification of theitem 11. The desired divert location is determined locally on the card20 and the item 11 is either allowed to continue on or is diverted toone of two other conveyors completing a 90 degree right angle transfer.The card 20 processes the timing required to ensure that the item 11 iscompletely transferred prior to initiating the next decision relating toa subsequent item transfer. The card 20 further processes any additionalinput/output required to signal the transfer device.

Another configuration is a “T” transfer as shown in FIG. 11G. Thispre-programmed operation facilitates an item transfer from one conveyorzone to one of two possible divert points. An identification device,e.g., a bar code scanner or RFID, is connected directly to acorresponding card 20 and provides the method of identification of theitem 11. The desired divert location is determined locally on the card20 and the item 11 is either allowed to continue on or is diverted toanother conveyor completing a 90 degree right angle transfer. The card20 processes the timing required to ensure that the item 11 iscompletely transferred prior to initiating the next decision relating toa subsequent item transfer. The card 20 further processes any additionalinput/output required to signal the transfer device.

The illustrated embodiment includes either a display on the card 20 or atouch screen type interface on a personal computer or industrialcomputing device. The touch screen allows a setup operator to easilydrag and drop the aforementioned pre-programmed operations to each card20, with no knowledge of programming ladder logic or other programmingrequired. The interface also allows parameters such as speed, timing,direction, etc. to be easily communicated to the individual cards 20 viathe network.

The illustrated embodiment of the conveyor system 10 also has theability to retain a backup of each card 20 in the system, should areplacement be required because of a card failure. The parameters andstandard code blocks can simply be downloaded to the card 20, greatlyminimizing downtime in the event of a card failure.

As discussed above, it is not desirable to have gaps 30 between items 11on a conveyor system 10. The gap control system of the present inventionuses MDR rollers in conjunction with cards 20 and sensors 14 to controlthe gaps 30 between items 11. As shown in FIG. 5 , for example, in theillustrated embodiment, any item 11 being conveyed is greater in length“L” (measured longitudinally along the direction of flow of theconveyor) than the distance “d” between the zone sensors 14. The sensors14 are preferably photo-eye type sensors, but any type of sensor capableof determining the presence of an item 11 could be used, including, butnot limited to, proximity sensors, limit switches, strain gauges, weightmeasurement devices, imagers, or ultrasonic sensors.

Depending on the nature of the items 11 being conveyed, the size of thezones 15 may be smaller than the arrangement shown in FIG. 1 .Accordingly, the spacing of sensors 14 may be varied to accommodateitems 11 of various sizes. Referring to FIG. 6 , items 11 of length “L”must be greater than distance “d,” the distance between sensors (andalso the length of the zone), in other words, the length of zone 15, inorder to effectively eliminate gaps between items. Alternatively, wherethe item length “L” is less than distance “d,” in some circumstances itmay not be possible to effectively eliminate the gap between an upstreamitem and a stopped downstream item depending upon the location at whichthe downstream item is stopped within the downstream zone. The zonelength can be varied as desired depending on the size of the items to beconveyed. Thus, the zone length may be greater than the item length orless than the item length as desired.

FIGS. 4-8 show one embodiment of the conveyor system 10 featuring thegap control system of the present invention. FIGS. 9-10 show a secondembodiment of a conveyor system 10 featuring the inventive gap controlsystem.

The illustrated embodiment uses brushless DC motors integrated withinthe MDRs. These motors provide feedback as to the position of theroller, e.g., via Hall effect sensors that are integral with the motoritself. The inventive gap control system utilizes the feedback from theHall effect sensors to dynamically determine the length of the gap 30between items 11 that are transported on the conveyor system 10.

More specifically, as an item 11 passes a sensor 14, the sensor 14transitions from blocked (i.e., when the item 11 is triggering thesensor 14) to unblocked (i.e., when the item 11 is not triggering thesensor 14). Accordingly, as an item 11 passes a sensor 14, a counter isinitiated within a microprocessor 28 on the card 20. The counter countsthe number of pulses received by the Hall effect sensor within an MDRuntil a subsequent item 11 blocks the sensor 14. In other words, thecounter records the number of pulses received from the MDR while no item11 was being transported by the MDR, which is a measure of the gap 30between consecutive items 11. A counter value is calculated andmaintained for every sensor 14 in the conveyor system by correspondingcards 20. Thus, as a downstream item 11 approaches a blocked zone 15,the corresponding counter value is used to continue to transport an item11 the requisite number of pulses, i.e., the number of pulses stored inthe counter value, in order to close the gap 30. This effectivelytransports the item 11 right up to the stopped item 11, barely touchingit, providing zero pressure accumulation as desired.

In addition to the scenario discussed above, i.e., wherein a sensor 14detects a second item 11 after a first item 11 has passed the sensor 14,there is another scenario in which the counter may be stopped.Specifically, the second scenario occurs when an item 11 that has passeda particular sensor 14 is stopped on the conveyor system 10 before asecond item 11 is detected by the particular sensor 14. In other words,the item 11 is stopped before the second item 11 reaches the particularsensor 14. Thus, when the item 11 has been stopped, the card 20corresponding to the downstream zone 15 in which the item 11 is stoppedcommunicates with the card 20 corresponding to the adjacent upstreamzone 15, which then stops the counter. Accordingly, the counter valuerepresents the distance between the end of the stopped, downstream item11 and the sensor 14 in the adjacent, upstream zone 15. Thus, the cards20 communicate with one another such that a subsequent item 11 is movedup to the stopped item 11, thus closing the gap 30 between the twoitems.

The illustrated embodiment also allows the counter value to be convertedto a known distance, such that the counter value could be modified toallow the user to select a desired gap distance between items 11 so thatitems 11 do not touch, but have a minimal gap 30.

The present invention further includes a method for determining a gap 30between items 11 on a conveyor system 10. Generally speaking, as shownin FIG. 12 , the method includes the steps of (a) detecting a first edgeof a first item at a predetermined location on a conveyor system; (b)detecting a second edge of the first item at the predetermined locationon the conveyor system; (c) beginning a counter once the second edge ofthe first item passes the predetermined location on the conveyor system;(d) stopping the counter once a first edge of a second item is detectedat the predetermined location; and (e) generating a counter value basedupon the counter data. Accordingly, the counter value represents a gap30 between adjacent items 11 on a conveyor system 10.

Additional steps of the method may include using the counter value toeliminate the gap 30 between the items 11. For example, the countervalue may be used to rotate an MDR a corresponding number of rotationsin order to close the gap 30 between the adjacent items 11 in the eventthat the downstream item 11 has been stopped on the conveyor system.Moreover, the counter value may be adjusted or converted to apredetermined value so that the size of the gap remains fixed if a gap(preferably minimal) between items 11 is desired.

The method may also include steps to stop the counter when an item hasbeen stopped in a downstream zone. For example, the method may includethe steps of (a) detecting a first edge of a first item at apredetermined location on a conveyor system; (b) detecting a second edgeof the first item at the predetermined location on the conveyor system;(c) beginning a counter once the second edge of the first item passesthe predetermined location on the conveyor system; (d) stopping thecounter when the item has been stopped on the conveyor system; and (e)generating a counter value based upon the counter data. Further stepsmay include using the counter data to rotate an MDR a correspondingamount of rotations in order to close the gap 30 between the adjacentitems 11.

Various alternatives and modifications are contemplated as being withinthe scope of the following claims particularly pointing out anddistinctly claiming the subject matter regarded as the invention.

1-20. (canceled)
 21. A motorized drive roller conveyor comprising: oneor more networked cards, each card configured to control one or moreadjacent zones, each zone comprising a motorized drive roller assemblyand a sensor configured to detect items on the conveyor, wherein the oneor more networked cards are configured to: measure a gap between a firstitem and a second item positioned consecutively on the conveyor bybeginning a counter when a trailing edge of the first item passes thesensor and stopping the counter when a leading edge of the second itempasses the sensor to generate a counter value indicating a length of thegap, and in response to the first item being stopped, advance the seconditem a distance derived from the counter value to adjust the length ofthe gap.
 22. The motorized drive roller conveyor of claim 21, whereinthe gap is measured by counting the rotations of a drive roller.
 23. Themotorized drive roller conveyor of claim 22, wherein the rotations ofthe drive roller are counted by counting the rotations of an integratedmotor in the drive roller.
 24. The motorized drive roller conveyor ofclaim 23, wherein the rotations of the integrated motor in the driveroller are counted using a Hall effect sensor.
 25. The motorized driveroller conveyor of claim 21, wherein the distance is equal to the lengthof the gap in order to eliminate the gap.
 26. The motorized drive rollerconveyor of claim 21, wherein the distance is less than the length ofthe gap in order to adjust the length of the gap to a predeterminedlength.
 27. The motorized drive roller conveyor of claim 26, wherein theone or more networked cards are further configured to receive user inputto define the predetermined length.
 28. The motorized drive rollerconveyor of claim 21, wherein the one or more networked cards arefurther configured to: stop the counter when the first item is stoppedin an adjacent downstream zone, prior to the leading edge of the seconditem passing the sensor, to generate the counter value; and cause themotorized drive roller assembly of an adjacent upstream zone to advancethe second item a distance derived from the counter value.
 29. Themotorized drive roller conveyor of claim 28, wherein there two or moreadjacent downstream zones.
 30. The motorized drive roller conveyor ofclaim 21, wherein each zone is shorter in length than a length of thefirst and second item.
 31. The motorized drive roller conveyor of claim21, wherein the counter value is converted into a standardized unit ofmeasurement.
 32. The motorized drive roller conveyor of claim 27,wherein the user input defines the predetermined length in units ofrotation of the drive roller.
 33. The motorized drive roller conveyor ofclaim 27, wherein the user input defines the predetermined length inunits of rotation of an integrated motor in the drive roller.
 34. Themotorized drive roller conveyor of claim 27, wherein the user inputdefines the predetermined length into a standardized unit ofmeasurement; and wherein the one or more networked cards are furtherconfigured to: convert the user input into a second unit of measurementassociated with the counter.
 35. A motorized drive roller conveyorcomprising: a first zone, a second zone, and third zone, wherein thesecond and third zones are two diversion points downstream of the firstzone, each zone including a drive roller and a sensor; and one or morecards comprising: an identification device that identifies a destinationzone for an item, a first driver that controls the first zone, a seconddrivers that controls the second zone, and a third drivers that controlsthe third zone, the one or more cards configured to: measure a gapbetween a first item, identified to divert to the second zone, a seconditem, identified to divert to the third zone, and a third item,identified to divert to the second zone, positioned consecutively on theconveyor by beginning a counter when a trailing edge of the first itempasses a first sensor, in the first zone, and recording a first countervalue indicating a length of a first gap when a leading edge of thesecond item passes the first sensor and recording a second counter valueindicating a length of a second gap when a leading edge of the thirditem passes the first sensor, detect the trailing edge of second item inthe third zone by the second sensor in the second zone, in response tothe first item being stopped, advance the third item a distance derivedfrom the second counter value to adjust the length of the gap.